Photosynthetic and Photoprotective Responses to Steady-State and Fluctuating Light in the Shade-Demanding Crop Amorphophallus xiei Grown in Intercropping and Monoculture Systems

Photosynthetic and photoprotective responses to simulated sunflecks were examined in the shade-demanding crop Amorphophallus xiei intercropped with maize (intercropping condition) or grown in an adjacent open site (monoculture condition). Both intercropping leaves and monoculture leaves exhibited very fast induction responses. The times taken to achieve 90% maximum net photosynthetic rate in intercropping leaves and monoculture leaves were 198.3 ± 27.4 s and 223.7 ± 20.5 s during the photosynthetic induction, respectively. During an 8-min simulated sunfleck, the proportion of excess excited energy dissipated through the xanthophyll cycle-dependent pathway (Φ(NPQ)) and dissipated through constitutive thermal dissipation and the fluorescence (Φ(f, d)) pathway increased quickly to its maximum, and then plateaued slowly to a steady state in both intercropping and monoculture leaves. When the illumination was gradually increased within photosystem II (PSII), Φ(NPQ) increased quicker and to a higher level in monoculture leaves than in intercropping leaves. Relative to their monoculture counterparts, intercropping leaves exhibited a significantly lower accumulation of oxygen free radicals, a significantly higher content of chlorophyll, and a similar content of malondialdehyde. Although monoculture leaves exhibited a larger mass-based pool size of xanthophyll cycle [V (violaxanthin) + A (antheraxanthin) + Z (zeaxanthin)] than intercropping leaves, intercropping leaves had a higher ratio of (Z + A)/(V + Z + A) than monoculture leaves. intercropping leaves had markedly higher glutathione content and ascorbate-peroxidase activity than their monoculture counterparts. Similar activities of catalase, peroxidase, dehydroascorbate reductase, and monodehydroascorbate were found in both systems. Only superoxide dismutase activity and ascorbate content were lower in the intercropping leaves than in their monoculture counterparts. Overall, the xanthophyll cycle-dependent energy dissipation and the enzymatic antioxidant defense system are important for protecting plants from photooxidation in an intercropping system with intense sunflecks.